ABSTRACT. This project is built around years of collaborative work between Drs. Murry and Regnier studying human embryonic stem cell derived cardiomyocytes (hESC-CMs) as a potential cell replacement strategy for cardiac repair following myocardial infarction (MI). Our group has shown that hESC-CMs and human inducible pluripotent SC-CMs (hiPS-CMs) can be produced at a scale and purity that permit testing in rodent models and the animal most likely to predict the human response: the non-human primate (NHP; Macaca nemestrina). We have demonstrated the ability of these cells to engraft in rodent models, covering the entire scar, and electrically integrate with host tissue to improve left ventricular performance. This project is based on two fundamental discoveries: 1) 2-deoxy ATP (dATP) is a potent natural nucleotide stimulant of cardiac contractility (via improved myosin binding to actin & faster detachment after the power stroke), and 2) hiPSC-CMs that overexpress the rate-limiting enzyme for dATP synthesis, ribonucleotide reductase (RNR), have both increased contractility and deliver dATP to the rest of the heart via gap junctions. Thus we will test the hypothesis that engineering hiPSC-CMs to elevate RNR (RNR-hiPSC-CMs) will improve outcomes in cell replacement therapy for MI (compared with control hiPSC-CMs), improving contractility of both graft and native myocardium. There are several highly novel aspects to our approach. 1) It is the first proposed use of cellular nucleotide manipulation to improve in vivo cardiac function. 2) The approach is not limited to replacement of lost tissue (with hiPSC-CMs) with a better functioning graft, but may also substantially benefit the post-MI depressed function of native myocardium. 3) The first use of engineered hiPSC-CMs to deliver what is effectively a small molecule therapy (dATP), a natural compound that improves heart muscle contraction. This effectively makes hiPSC-CMs a drug delivery device with cardiac specific delivery and effects. Aim 1 will develop and test engineered mutations in RNR that increase it's stability and activity in cardiomyocytes and their ability to titrate increasing levels of dATP produced in hiPSC-CMs. Aim 2 will use AAV vectors for RNR variants, selected from Aim 1, to investigate their capacity to improve cardiac function in a mouse model of myocardial infarct and heart failure. Aim 3 will produce engineered hiPS cell lines that will act as dATP `donor cells' following differentiation, for transplantation into acute MI and more challenging chronic MI athymic rat models to determine their capacity to improve function beyond transplantation of non- engineered hiPSC-CMs. We will evaluate the persistence of these effects and determine the long-term stability and viability of these cell lines. We expect significant contractile improvement of both the graft and native myocardium with RNR-hiPSC-CMs vs. hiPSC-CMs and this effect will be modulated by the dATP producing capacity of the transplanted cells. Results from these studies will elucidate the potential of this combination cell- and small molecule therapy to ameliorate or even improve pump function in failing hearts.